With preorders for Apple's next-generation iPhone beginning tonight, AppleInsider takes a look at the precautions iPhone owners planning to sell or trade their old handset for a brand-new iPhone 6 or iPhone 6 Plus need to take before divesting themselves of their passé device.

As iPhone resellers dump existing stock in preparation of an impending next-generation refresh, U.S. mobile carrier T-Mobile will supposedly cut up to $50 off device-only pricing in an across-the-line sale, according to leaked internal documents.

Apple has announced that it will replace potentially dangerous 5-watt European USB power adapters that were shipped with the iPhone 3GS, iPhone 4, and iPhone 4S, as well as sold separately from October 2009 to September 2012.

In an effort to increase its supply of certain iPhone models during the month of June, electronics buy-back service Gazelle is offering to pay an additional $20 on all working iPhone 4 and iPhone 4s trade-ins for the next 7 days.

Apple on Friday launched an iPhone upgrade promotion that sweetens the deal for iPhone owners looking to step to a new iPhone 5s or 5c. Specifically, owners of iPhone 4S models can net greater trade-in incentives than usual -- up to $199.

FreedomPop, which recently launched a cellular service to go along with its "free mobile data" plans, will be getting its first batch of Apple handsets with the iPhone 5 to sell as part of the firm's "freemium" service.

Along with the addition of headline features like CarPlay, Apple's latest iOS 7.1 update brought a welcome speed boost for the four-year-old iPhone 4, as well as new iBeacon handling that allows apps to search for signals even when closed.

A report on Tuesday claims Apple has returned the discontinued iPhone 4 to production, though the three-generation-old handsets will be available only to very specific markets in India, Indonesia and Brazil.

The evolution of Apple's iPhone — from the original model through the new 5s and 5c — was shown off recently in a new video that pits all eight released versions of the bestselling handset against each other in a test of speed.

This fall, Apple's newly revamped iPhone lineup is expected to include the high-end "iPhone 5S," a plastic "iPhone 5C," and a low-end option with the legacy iPhone 4S, according to a well-connected insider.